Conventionally, an electric latent image or a magnetic latent image is visualized by a toner in an electrophotographic device, an electrostatic recording device and the like. For example, in electrophotography, an electrostatic image (latent image) is formed on a photosensitive member, and a latent image is developed with a toner, whereby a toner image is formed. The toner image is generally transferred to a recording medium such as paper, and then fixed by a method such as heating. The toner used for developing electrostatic images is generally colored resin particles containing a colorant, a charge controlling agent and other additives in a binder resin.
As a fixing system in a dry developing system, a thermal heat roller system is widely and generally used for its fine energy efficiency. Furthermore, low-temperature fixing of toners has been demanded in recent years so as to lower the heat energy provided to toners during fixing for energy saving. It is considered that an essential technical matter to be achieved for attaining this demand is to lower the melting initiation temperature of a toner to thereby lower the fixing temperature.
Furthermore, since the improvement of fixing devices has been further improved, heat energy efficiency can be increased by decreasing the thickness of a roller on a side that is to be brought into contact with a toner image, and thus it is possible to significantly shorten a start-up time. However, since the specific heat capacity has been decreased, the difference in temperature between a part where a recording medium has passed and a part where a recording medium has not passed increases, and thus the adhesion of a toner to a fixing roller occurs. Therefore, a so-called hot offset phenomenon occurs, in which a toner is fixed on a non-image part on a recording medium after one rotation of a fixing roller. Therefore, the demands for the hot offset resistance together with the low-temperature fixability of toners have become stricter.
It is essential to incorporate a release agent (a softening agent) in a toner so as to improve the hot offset resistance of the toner, and properties such as low melting viscosity and excellent separability from resins are desired for such release agent. Generally, as release agents used for toners, for example, hydrocarbon-based waxes as represented by carnauba wax, polyethylene, polypropylene, paraffin and the like are known.
Meanwhile, a toner using a synthetic ester wax as a release agent has also been suggested. For example, Patent Literature 1 discloses a toner for developing electrostatic images containing at least a binder resin, a colorant and an ester wax, wherein the toner contains a specific amount of a specific ester wax, and also discloses that the transparency of a fixed image on an OHP film is improved, and that the toner is excellent in fixability and offset resistance. Patent Literature 2 discloses a toner containing a binder resin, a colorant and a release agent, wherein the release agent contains a monoester compound and hydrocarbon compound having specific structures, and also discloses that the toner can be fixed at a low temperature and does not cause band-like or string-like image defects in a fixed image.
Patent Literature 3 discloses a toner containing a release agent, a binder resin and a colorant, wherein the release agent has a kinetic viscosity, a melting point and the like within specific ranges, and also discloses that the toner is excellent in low-temperature fixability and fouling resistance. Patent Literature 4 discloses a toner having toner particles containing a binder resin, an ester wax and a colorant, which has components as detected at a specific time measured by a GC/MS analysis of the ester wax within specific ranges, and also discloses that the toner shows a fine fixing property even in high-speed image formation, suppresses machine fouling, and can provide an image without gloss unevenness for a long period. Patent Literature 5 discloses a toner produced by emulsifying or dispersing a liquid in which a toner material containing a binder resin and a release agent is dissolved or dispersed in an organic solvent, in an aqueous medium, wherein a mixture containing a synthetic ester wax formed of a monoester having a specific melting point and an erythritol wax having a branched structure, and a hydrocarbon wax having a specific melting point at a specific ratio is used as the release agent, and also discloses that the toner is excellent in release property and low-temperature fixability, and has low fouling property.
However, in accordance with the demands for energy saving in recent years, there were some cases when the balance of lowering a fixing temperature and heat-resistant shelf stability was insufficient in the toners obtained by the methods of above-mentioned patent literatures.